Why does solute lower freezing point

Next Unit Chemical Reaction Rates. Kendal Orenstein. Thank you for watching the video. Start Your Free Trial Learn more. Kendal Orenstein Rutger's University M. Explanation Transcript Freezing point depression is a colligative property of solutions. Chemistry Chemical Solutions. Science Biology Chemistry Physics. English Grammar Writing Literature.

All Rights Reserved. At a given temperature, if a substance is added to a solvent such as water , the solute-solvent interactions prevent the solvent from going into the solid phase.

The solute-solvent interactions require the temperature to decrease further in order to solidify the solution. A common example is found when salt is used on icy roadways. The de-icing of planes is another common example of freezing point depression in action. A number of solutions are used, but commonly a solution such as ethylene glycol, or a less toxic monopropylene glycol, is used to de-ice an aircraft.

The aircrafts are sprayed with the solution when the temperature is predicted to drop below the freezing point. The freezing point depression is the difference in the freezing points of the solution from the pure solvent. This is true for any solute added to a solvent; the freezing point of the solution will be lower than the freezing point of the pure solvent without the solute.

Thus, when anything is dissolved in water, the solution will freeze at a lower temperature than pure water would. The freezing point depression due to the presence of a solute is also a colligative property. That is, the amount of change in the freezing point is related to the number of particles of solute in a solution and is not related to the chemical composition of the solute.

Recall that covalent and ionic compounds do not dissolve in the same way. Ionic compounds break up into cations and anions when they dissolve. Covalent compounds typically do not break up. Remember that colligative properties are due to the number of solute particles in the solution. Adding 10 molecules of sugar to a solvent will produce 10 solute particles in the solution. Colligative properties depend on the number of solute particles in the solution.

By knowing the molality of a solution and the number of particles a compound will dissolve to form, it is possible to predict which solution in a group will have the lowest freezing point. The presence of a solute lowers the freezing point of any solvent ; this effect is called freezing-point depression.

The key to understanding this effect is that the solute is present in the liquid solution, but not in the pure solid solvent. Example: think of pure ice cubes floating in salt water. At the freezing point of any solution the rates of melting and freezing are exactly balanced, allowing both phases to coexist in a thermodynamically stable state.

The introduction of a solute reduces the activity of the liquid phase solvent, thereby reducing the rate of freezing. You can think of this reduction in activity as solute molecules "getting in the way" of solvent molecules from attaining the correct alignment for freezing at the surface.

First, it must not contribute to the vapor pressure of the solution, and second, it must remain suspended in the solution even during phase changes. Because the solvent is no longer pure with the addition of solutes, we can say that the chemical potential of the solvent is lower. Chemical potential is the molar Gibb's energy that one mole of solvent is able to contribute to a mixture. The higher the chemical potential of a solvent is, the more it is able to drive the reaction forward.

Consequently, solvents with higher chemical potentials will also have higher vapor pressures. The boiling point is reached when the chemical potential of the pure solvent, a liquid, reaches that of the chemical potential of pure vapor.

Because of the decrease in the chemical potential of mixed solvents and solutes, we observe this intersection at higher temperatures. In other words, the boiling point of the impure solvent will be at a higher temperature than that of the pure liquid solvent.

Thus, boiling point elevation occurs with a temperature increase that is quantified using. Freezing point is reached when the chemical potential of the pure liquid solvent reaches that of the pure solid solvent. Again, since we are dealing with mixtures with decreased chemical potential, we expect the freezing point to change. Unlike the boiling point, the chemical potential of the impure solvent requires a colder temperature for it to reach the chemical potential of the pure solid solvent.